Biodegradable polyurethanes and polyurethane ureas are typically formulated using polyester polyols, aliphatic diisocyanates and diol or diamine chain extenders. The polyester polyol forms the ‘soft’ segment of the polymer while the diisocyanate and the chain extender form the hard segment. The hard segment forms ordered domains due to hydrogen bonding and imparts high mechanical strength to the material. The soft domains are formed largely by the polyester polyol and provides elastic properties to the polymer. Polyester polyols such as polycaprolactone, polyglycolide and polylactide are the most widely used polyols in biodegradable polyurethanes. The biodegradation of these polymers occur largely due to the hydrolytic degradation of the ester, urethane and urea linkages of the polymer. The soft segment of the polyurethane degrades significantly faster than the hard segment. This is largely due to the presence of relatively easily hydrolysable ester linkages and the amorphous nature of the soft segment. The hard segment of biodegradable polyurethanes is formed from diisocyanates such as hexamethylene diisocyanate (HDI), butane diisocyanate (BDI), lysine diisocyanate ethyl ester and lysine diisocyanate methyl ester. The chain extenders are low molecular weight (typically MW<400) diols or diamines. Examples include 1,4-butanediol, ethylene glycol, ethylene diamine and water. The diols and diisocyanates react to form urethane linkages in the hard segment of the polyurethane. The diamine chain extenders and water react to form urea linkages. The urethane or urea linkages in the hard segment also degrade by hydrolysis but at a significantly slower rate than ester linkages.
An important consideration in the design of biodegradable polymers is the choice of precursors that would lead to polyurethanes with backbone functional groups susceptible to one or more degradation pathways in the body, such as hydrolytic or enzymatic degradation. Such polyurethanes degrade to low molecular weight products which are either bioresorbed or released from the body through one of the waste disposal pathways in the body. The use of conventional diisocyanates and chain extenders such as ethylene glycol or ethylene diamine leads to polyurethane with hard segments with urethane, urea or a combination of such functional groups. Because of the relatively slow degradation rates of these linkages compared with ester linkages, the polymer degradation may lead to oligomers containing mainly hard segments. This becomes a major concern, particularly when polyurethanes are formulated with a higher percentage of hard segment (longer hard segment lengths). Accordingly, it is desirable if the hard segments also break down rapidly to low molecular weight compounds for rapid release from the body. This also broadens the formulation options for the design of biodegradable polyurethanes with degradation rates tailored to specific applications.
Chain extenders which break down to biocompatible compounds such as amino acids have been used for formulating biodegradable polyurethanes. The chain extenders are diamines based on cyclohexane dimethanol and phenyl alanine and are generally too high in molecular weight (MW 438) to be considered as chain extenders. The high molecular weight combined with the bulky benzyl pendant groups leads to polyurethanes with disrupted hard segments, limiting the range of properties that can be achieved using such chain extenders in polyurethanes.